Race 4 Fusarium: SJV Pima pariah

Its official name is Race 4 FOV (Fusarium oxysporum f. sp. Vasinfectum); Fusarium for short.

Its unofficial names bestowed by San Joaquin Valley cotton producers are pariah, leper, outcast or untouchable.

It is like your ugliest cousin you don’t want anyone to see. It is so ugly there are SJV cotton growers who want to keep it hidden in the closet, so no one will find it and for good reason. If it gets well-established in a field, it can render that field worthless for Pima cotton production and may actually reduce the land’s value as a similar fusarium has in Australia.

You might as well put a skull and cross bones across any for sale sign stuck into the ground of a Race 4 Fusarium-infected field.

Race 4 fusarium fungus devastates all but one Pima cotton variety. Fortunately, it is the most widely planted variety in the valley, Phytogen 800. However, even it has not escaped the stigma of Race 4 Fusarium. Once it was called resistant. Now it is called tolerant.

Where Race 4 came from, no one will say for sure. Its most likely source was from planting seed production. Whose seed? No one is talking. There is some speculation it may have come from seed stock imported from another country.

It has become the most feared and devastating problem to confront the SJV cotton industry since verticillium wilt devastated cotton on the East Side of the San Joaquin Valley about 35 years ago. Eventually, verticillium wilt resistance was bred into valley Acala varieties and the problem subsided. Breeding is the only way to derail Race 4 fusarium. Public and private breeders are working feverishly to find additional resistant or tolerant Pima varieties. Race 4 can be a problem in Acala cotton, but not nearly as devastating as in non-tolerant Pimas.

Without a stable of Race 4 resistant varieties, researchers have developed containment strategies.

Race 4 could not rear its ugly head at a more inopportune time.

Pima cotton is the future of the San Joaquin Valley cotton industry and Race 4 Fusarium devastates Pima. It was identified several years ago when researchers were looking for a similarly devastating Australian fusarium some feared had been brought into the valley on imported Australian cottonseed for the dairy industry. Researchers never found the Australian fusarium race. However, they found a brand new California fusarium, Race 4.

Like its Australian cousin, Race 4 is not associated with nematodes, a vector of another fusarium, Race 1, which has been in California for years, but is seldom a major problem.

The old fusarium is associated with light soils and nematodes. Race 4 is associated with heavier soils and needs no nematodes to spread around. It is spread by infected soil and water.

It has been a gnarly fungal disease for University of California extension cotton specialist Bob Hutmacher and UC Davis plant pathologist Mike Davis and others to research. Davis was the first to identify the new race.

It is such a pariah that many growers who suspected they had fusarium refused to allow UC researchers on their farm for fear of word getting out. Some did eventually, swearing researchers to secrecy.

Eventually more growers let Hutmacher and others conduct research in fusarium infected fields as it became apparent the problem was more widespread than many believed. Hutmacher continually praises those cooperators.

Hutmacher detailed much of his findings recently at the annual cotton field day at the UC Westside Research and Extension Center at Five Points, Calif.

The only way to positively identify Race 4 is through soil samples, Hutmacher explained.

However, the state cotton specialist said growers can sample suspected Fusarium spots in Pima in the early spring. Later in the year, it can be confused with verticillium.

Blank areas in field devoid of plants or dying plants early are signs of possible Race 4 Fusarium. First-year infestations are typically a few infected plants or a few feet of row while second and third year blank areas can expand to the size of small or medium rooms as inoculum spreads.

Surviving plants around the blank areas will be wilted, splotchy with foliar chlorosis and/or necrosis and nearly continuous, dark brown vascular staining in the tap roots.

By comparison, verticillium appears later in the season, mid-to-peak bloom and later. Verticillium vascular staining is also generally lighter in color.

If the problem is caught early enough, Hutmacher recommends roguing infected plants out of the field.

“Do not bury infected plants if possible. Pull up plants and burn them or compost them at high temperatures,” he said.

Hutmacher also recommends identifying the infected area with a hand-held GPS unit to monitor seasonal changes in the size of the affected area.

Race 4 spores germinate when host plant roots are infected. It colonizes dead tissue and survives on plant debris and in the soil. It also can survive on roots and tissue of many weeds. Spores can survive for years in the soil, noted Hutmacher.

“Race 4 becomes a permanent soil resident once introduced and cannot be eradicated,” Hutmacher said.

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